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That seems a bit counter-intuitive, because in an orbit above the belt, the satellite would eventually lose gain velocity, so its orbit would intersect the orbital plane of the geostationary satellites possibly creating a collision hazard. To be re-orbited, a satellite has to have functional attitude control [Wikipedia], which would also allow to lower increase the velocity to decrease the altitude instead. So, why isn't the so-called graveyard orbit below the geostationary altitude?

Mark
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  • Losing velocity happens when you raise an orbit. Lower orbits are faster. The moon is slowing down and therefore leaving Earth's orbit some day. You are probably thinking of low Earth orbit (LEO) where there is atmospheric drag. Despite the counterintuitive feeling it gives, drag causes LEO satellites to orbit faster as they fall to Earth. Give them a boost, they raise to higher orbit, but orbit more slowly. – uhoh Apr 07 '17 at 17:00
  • @uhoh: The Moon orbit growing is caused by tidal bulging of Earth due to Moon gravity. The satellites are not nearly massive enough to cause such effect. (never mind the effect is exactly zero at GEO, reversing direction there.) – SF. Apr 07 '17 at 17:17
  • @SF. that's right but I didn't mix the two - just giving supporting examples of things that slow down by moving away, or move away by slowing down. The cause doesn't matter, it's just the law. https://i.stack.imgur.com/ZJqpE.jpg – uhoh Apr 07 '17 at 17:27
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    Yes. Indeed - Lower orbit = higher velocity. My bad. Thanks @uhoh. – Mark Apr 07 '17 at 17:47
  • Not "bad" at all! Actually it's really cool and caught me by surprise. See this which draws from this. – uhoh Apr 07 '17 at 17:56
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    @uhoh, I knew that, yet made this silly mistake :) – Mark Apr 07 '17 at 18:02
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    Lower orbit has higher velocity but to go from a high orbit to a low orbit is done with a pair of deceleration burns. Welcome to the strange world of orbital mechanics. – Peter Green Apr 07 '17 at 18:50
  • @uhoh The moon's orbit is expanding as it gains angular momentum and energy by tidal transfer, but it will not escape as there is isn't enough energy in the system. If the Sun wasn't eventually going to boil the system away the eventual fate would be a tidal lock. – dmckee --- ex-moderator kitten Apr 07 '17 at 18:55
  • @dmckee whew! that's great news! Does energy run out sooner than angular momentum? – uhoh Apr 07 '17 at 19:01
  • @dmckee: Source? I had asked that question on Astronomy.SE and the response was... disappointing. – SF. Apr 07 '17 at 23:46
  • @SF Well, I admit I only checked my memory with Wikipedia, and I don't have their references to follow-up. I'll think about it. – dmckee --- ex-moderator kitten Apr 08 '17 at 00:21

2 Answers2

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Orbits at the altitude of GEO are stable for very long times (millions of years). There is no significant decay of the orbital height due to some kind of drag, so the risk of these satellites interfering with working ones is close to zero. On the other hand, there are good reasons to store them above the belt and not below:

The region below is used for maneuvers, e.g. to move a satellite from one longitude to another (ok, not a really good argument, because you could do this above as well).

One very important aspect: If satellites are in a lower orbit, they might interfere with communication links between GEO and Earth if the satellite passes by directly in front of another.

There are two ways how satellites typically approach GEO: from a high GTO using conventional thrusters, so they move rather quickly and chance of collisions are low. On the other hand, there are some satellites using ion thrusters to reach their final orbit rather slowly and from lower orbits. Having to move them through the graveyard orbit would be a great risk due to the long travel time.

Additional information: Have a look to this question: Why is the ribbon of decommissioned geosynchronous satellites skewed? - the graveyard orbits are not strictly above GEO, but also tilted by several degrees.

Community
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asdfex
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Actually, it makes a lot of sense to raise the orbit of end-of-life geostationary satellites: Coming from Earth you have to cross through a lower orbit to transfer from low earth orbit to a geostationary orbit but you don't have to go farther out than that (some transfer orbits do, but it's not a requirement). That means that a higher orbit has less risk of collision than a lower orbit with regard to the continuing use of the geostationary orbit belt.

At the altitude of geostationary satellites (~36,000 km) there is basically nothing to cause orbits to lower on a reasonable time scale. The atmosphere doesn't stop at a set altitude (the exosphere is apparently measurable up to 10,000+ km and estimated to reach half way to the moon), but the effect of drag is minuscule that far out. I didn't find a good reference for the longevity of geostationary orbits specifically, but LAGEOS is only at 6,000 km and is estimated to not reenter for 8.4 million years.

The intended graveyard orbit, as required by US regulations for geostationary satellites, is 300 km above the geo belt. That's not very far compared to the distance of the geo belt from Earth, but it's more than far enough to ensure that maneuvers in the geo belt of active satellites and perturbations from the sun and moon on the dead satellites don't cause them to cross paths.

1337joe
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    the applicable NASA standard for orbital debris also calls for a graveyard orbit lower than GEO, but I do not know how frequently it is used, in practice – costrom Apr 08 '17 at 17:06